There has been an increase in the number of tick-borne infections reported in the United States over the last decade. Among the most threatening of these are the rickettsial infections. The best known of the human rickettsial infections is Rocky Mountain Spotted Fever (RMSF), a disease that maintains a fatality rate of 3-10%, even with modern medical care. The etiologic agent of RMSF is Rickettsia rickettsii, one of 18 recognized members of the Rickettsia Spotted Fever Group (SFG) and a NIAID Category C Priority Pathogen. Maryland reported 1,530 cases of RMSF from 1970-2006 with an average of 79 cases during each of the last five years. The majority of these cases were reported from the counties along the shore of the Chesapeake Bay, where human densities are highest. This region also supports large populations of the three most common human-biting ticks in the eastern United States, Dermacentor variabilis, Amblyomma americanum and Ixodes scapularis. Although D. variabilis is considered the primary vector of R. rickettsii, A. americanum has been incriminated as a secondary vector. Additionally, Rhipicephalus sanguineus also occurs throughout the region in association with domestic dogs. Although not a frequent human-biter, this species was recently identified as the vector for a cluster of cases in Arizona. Despite preliminary information illustrating that these ticks have a 3-4% infection rate for SFG Rickettsia, R. rickettsii has NOT been identified from these vectors. The Rickettsia that have been molecularly recovered from these ticks are species that are either considered to be non-pathogenic in humans, or of unknown identity. The serologic tests commonly used to confirm infections such as RMSF do not differentiate between members of the SFG rickettsiae and the specific pathogens causing clinical disease then become relative unknowns. It is therefore integral to public health to prove that R. rickettsii is not circulating in tick populations, and to identify what Rickettsia and Rickettsia-like organisms may be responsible for human clinical disease. The exploratory intent of this R21 application will allow us to prove the hypothesis that R. rickettsii is NOT circulating in endemic vector populations and will additionally allow us to acquire the preliminary identifications of organisms that may be responsible for human disease. This data will then be used to construct more comprehensive investigations of pathogen exposure, improved diagnostics, pathogen ecology and pathogenicity to human populations. This is extremely important as many potential pathogens that may fall under NIAID categorization have yet to be identified. Additionally, in the last few years, several laboratories have reported Rickettsia-like organisms from Ixodes ticks. The impact of these potential new pathogens to human health is currently unknown. Specific aims are to: 1. Demonstrate that Rickettsia rickettsii is not circulating in the classic Dermacentor cycle in eastern Maryland, a region considered endemic for Rocky Mountain Spotted Fever. 2. Identify and characterize Rickettsia and Rickettsia-like NIAID Priority Pathogens infecting man-biting ticks of Maryland.